Uno External Power AND USB Power - CAN Shield Question

Currently using an Arduino Uno for PWM output to control a MOSFET switching circuit for an actuator. The actuator requires more current than is safe to draw from the Arduino pins - therefore requiring an external power supply.

The external power supply will be 12-13.8v (from a vehicle) through a DC-DC 9v 600mA (max) regulator which will be connected to the Vin pin - with the full voltage split off to power the actuator. New configurations will have to be flashed on the fly with outputs being monitored in the serial output window. This requires a USB connection, in addition to the external power, which results in two power supplies simultaneously.

From other threads, it seems like this is not a problem for an Uno and that it will auto select the external power source as it's the higher voltage and allow for external power while continuous USB communication to a PC.

1) Although the Uno can handle it, will having two power sources cause problems with additional shields (Can-bus shield from seeed studio to be exact)?

2) If it will be a problem, could a USB cable be modified to only connect the data+/data- lines and either snip or block the voltage/ground pins to maintain USB data, but not power?

Hmm this makes sense.

I don't plan on pulling power from the OBDII port. So I could leave V_OBD off, and run dc cigarette outlet (12-13.8v -> 9v regulator -> power plug port on main arduino board. Then the CAN shield will inherently be powered when it's stacked onto the Arduino via Vin (or 5v) pin.